Method of electron-microscopically investigating subjects



M. PLOKE METHOD of' ELEcTRoN-MICROSCOPICALLY INVESTIGATING SUBJECTSV Oct. 22, A1940.

Filed OCT.. 2. 1937 'Patented O ct. 22, 1940 PATENT QFFICE METHOD OF -ELECTRON-MICRO SCOPIQALLY INVESTIGATIN G SUBJECTS Martin Ploke, Dresden, Germany, assignor to Zeiss Ikon Aktiengesellschaft, Dresden, Germany Application October 2, 1037, Serial No. 167,051

2 Claims.

The invention relates to improvements in electron-optics and particularly to electron-microscopy.

In electron-microscopy three methods have 1` been developed principally. According to one method th'e subject, consisting for instance of a filament, is observed in its own electron light, produced by rendering the filament incandescent. 'Ihe other two methods are applicable to subjects which do not emitelectrons and these methods are based on irradiation of the subject ,withy electrons or on passage of electrons through the subject to be investigated.

The present invention has the'object of avoiding the production of heat which accompanies the bombardment of an article with electrons.

For this purpose the invention has as its object a method of producing primarily an image oi the subject on a photo cathode, and then enlarging this image electronoptically.

The invention furthermore has the object of producing this enlargement of the cathode image by means of electrical or magnetical lenses.

The invention furthermore has the object of permitting in electron-microscopy that the subjects can be changed rapidly and without difculties, as contrasted with known methods in which the subject had to be positioned within a vacuum.

Another object of the invention is to project the image of the subject on the photo cathode by rays of Shortwave length, for instance by X-rays, avoiding eventually any projection by long wave light rays which are visible. The base on which the photo-electric layer of the cathode is positioned preferably consists of a material which stance, quartz.

permits the passage oi.' ultra-violet rays, as for in- When employing X-.rays it is feasible to use as a photo cathode thin metal foils of the type known as Lenard windows. Since the present invention requires a photo cathode,l or Lenard window respectively, of very small dimensions only, the cathode is eminently adapted to. withstand the outer air pressure.

In the method of the present invention is employed to produce very great enlargements, it may happen that the intensity of the light in which the image appears on the fluorescent screen of the electron microscope is very weak.

'I'he invention therefore also has the additional object of intensifying this illumination by using a process screen similar to screen plates known from lthe printing process in place of the fluorescentl Germany Gctober 2, 1936 fect, as each of the projecting point of the screen plate may be charged to a predetermined degree through concentration of the electrons emitted from the photo cathode on the projections of the plate. The electrical impulses produced by the charges are amplified and are used to control the electron beam oi a Braun tube on whose fluorescentscreen the electron-optically enlarged image will appear with sumcient brightness. 'I'he uorescent screen of the Braun tube may also be replaced by a screen consisting for instance of a great number of cells which change their lightpermeability under the influence of the electron beam striking the same.

The drawing illustrates by way of example vtwo embodiments of the invention.

Fig. 1 shows diagrammatically an arrangement in which the subject to be examined is illuminated by ultra-violet rays.-

Fig. 2 illustrates diagramm'atically a modified system in which the subject to be examined is exposed to X-rays, the electron image on a screen ,cathode being scanned, and the scanning impulses being utilized for controlling the electron beam of aBraun tube.' and n 25 Fig. 3 illustrates diagrammatically the transmission of the scanning impulses to a Braun tube. According to Fig. 1, the subject I is illuminated, as for instance a short length of filament, with ultra-violet rays produced by the high pressure mercury lamp 3 arranged in front of the parabolic reflector 2. A lens 4 or similar optical means projects a true im'age of the subject I onto the photo cathode 5 positioned on the interior face of a quartz window 6. The electrons emitted from 35 the photo cathode 5 and influenced by the magnetical lenses 1 and 8 produce upon the fluorescent screen 9 an enlarged image of the subject I. The fluorescent screen 9 is arranged upon the transparent cover plate I0 of the evacuated tube Il, which is provided in its interior with the anode I2. A voltage of about 50,000 volts is applied between the photo cathode 5 and the anode The modified embodiment of Fig. 2 employs an X-ray tube 20 in front ofv which a diaphragm 2|, preferably consisting of lead, is arranged. 'Ihe X-rays project an image of the subject 22 on a thin metallic window pane 23 which forms a closure on the reduced portion 24 of the evacuated tube 25. The X-rays cause the metallic foil 23 to emit electrons and these electrons are concentrated upon the cathode screen 29 owing to the influence of the anode 26 and the magnetical lenses 21 and 28. The electron image 55 appearing on the cathode screen 29 is scanned by an electron beam 30 produced in the scanning assembly 3| which comprises the customary electrode arrangement as indicated. The beam being osciliated by the defiecting plates 32 and deilecting coils 33 respectively. 'I'he charge impulses produced during the scanning are y employed for controlling the electron beam 34 of a Braun tube 35.

Fig. 3 shows dlagrammatically how this may be accomplished. This Fig. 3 shows the electrical circuit connections leading uniting the electrodes of the tubes 25 and 35. 'Ihe impulses taken from the screen cathode 29 are conducted to an amplier 31 and then to the control electrode 38 of the Braun tube. 'I'he elements indicated at 39 and 40 produce the oscillations on the deflecting plates 32 and the deflecting coils respectively of the tube 25 and the Braun tube 35,

What I claim is:

1. In a system for producing enlarged images of subjects, an electron tube comprising an elongated evacuated vessel having a reduced end portion, a metallic foil closing said reduced end portion and adapted to emit electrons when an image of the subject is projected on the same by X-rays, a cathode screen mounted within said vessel at the other end thereof, a tubular anode extending along the inner wall of said vessel from adjacent said metallic foil toward said cathode screen. means forming magnetical lenses mounted around said elongated vessel for concentrating the electrons emitted by said metallic foils, one of said magnetical lens means being mounted around the reduced end portion of said vessel and another one surrounding the said vessel at a place substantially midway between its ends, said anode and magnetical lens forming means being operable to project a magnied electron image of the subject on said cathode screen; said elongated vessel being provided with a tubular extension within which are mounted means for generating a ray of electrons and directing the same at said cathode screen, and means causing said ray to scan said cathode screen.

2. Ina system for producing enlarged images of subjects, an electron tube comprising an elongated evacuated vessel having a reduced-end portion, a metallic foil closing said reduced end portion and adapted to emit electrons when an image of the subject is projected on the same by rays of short wave length, a cathode screen mounted within said vessel at the -other end thereof, a tubular anode extending along the inner wall of said vessel from adjacent said metallic foil toward said cathode screen, means forming magnetical lenses mounted around said elongated vessel for concentrating the electrons emitted by said metallic foils, one of said magnetical lens means being mounted around the reduced end portion of said vessel and another 25 

